Digitizer tablets are well-known. See, for example, the review article in Byte, January 1989, pages 162-174. The most popular types employ electromagnetic technology of the so-called travelling-wave type exemplified by Kamm U.S. Pat. No. 3,904,822 and Zimmer U.S. Pat. No. 4,368,351. This type operates by sequentially exciting the conductors or wires of a regular grid to cause, in effect, an electromagnetic field or wave to travel along the tablet in the direction of one of its coordinate axes. The wave induces in a coil in a pointing device over the tablet an electrical signal which when processed peaks in one polarity before the coil and peaks in the opposite polarity after the coil. The coil center is located by measuring the time for the travelling wave to pass under the coil. This mode of operation requires separate pulsing of each wire of the grid, logic and counter circuits for the time measurement, and phase detector circuitry for detecting the phase change in the induced signal as it crosses the coil. The result is that considerable circuit components are required to achieve user requirements of resolution and accuracy. A reverse mode of operation excites the coil and then scans the wires in sequence to detect the phase change of the voltages induced in the wires. This tablet construction also requires excessive circuit components.
Many proposals have been made to reduce the required circuitry and thereby reduce the manufacturing cost of such devices. A principal goal has been to reduce the number of individual wires that have to be individually excited or scanned. For example, a popular 12 inch.times.12 inch tablet employs 64 active wire portions made up of 64 individual wires. To scan with a common detector or excite from a common pulse source 64 separate wires requires a 1.times.64 multiplexer (MUX), typically obtained by cascading one 1.times.4 MUX with four 1.times.16 MUXs, a total of five components. Reducing the number of active wire portions to reduce the number of components will, for a given size of table however, sacrifice resolution.